Carbonate slopes receive, lose, and store sediments through the interplay of deposition, erosion, cementation, and dissolution. The development of accretionary or erosive slopes mainly depends on the slope angle (Schlager and Camber, 1986). Increase in slope angle will shift the activity of gravity flows from depositional to erosional, and thus control the type of sedimentary deposits seen in this environment. In addition, the type of sediment stored on the slope will control the slope declivity.
Kenter (1990) clearly showed that carbonate muds are not able to maintain a slope that exceeds 5°, whereas coarse-grained deposits can build slopes with an angle of up to 32°. The grain size of the sediments produced on the platform will, therefore, play an important role in determining the type of slope that might develop.
The present-day morphology of the Bahamas shows shallow-water platforms that are separated from deep-water areas by relatively steep slopes. On the leeward side, the slope profile displays a 4-km-wide gently sloping interval between the platform interior and the slope break at 55-65 m water depth (Wilber et al., 1990). Seaward of the slope break, gradients increase to a water depth of 140-180 m, forming an almost vertical wall. At the base of this wall, a 30-m-wide trough is present. This is followed by a 100- to 200-m-wide depositional ridge that shoals for about 40 m. Westward of this elevation, the slope continues with angles between 15° and 20° (Wilber et al., 1990). Slopes in the Tongue of the Ocean (windward margin of the Great Bahama Bank) have a more regular topography (Grammer et al., 1991; Grammer and Ginsburg, 1992) whereby the profile shows a platform edge with angles ranging between 20° and 30° down to 60 m water depth, and a 60-120 m near-vertical wall (65°-90°), followed by a steeply inclined 32°-38° slope that gradually decreases to angles of 25°-28° down into the basin (Grammer et al., 1991; Grammer and Ginsburg, 1992).
Grammer et al. (1991) also showed that slope development on the windward margin differs from that on the leeward margin in the Tongue of the Ocean. The most striking feature is the variable thickness of the unconsolidated fine-grained sediments onlapping the cemented slope. The leeward slope displays some areas of larger sediment wedges onlapping the cemented slopes when compared with its windward counterparts. The extensive sediment wedge present on the western leeward side of the Great Bahama Bank fits into this model. High-resolution seismic profiles show large-scale export of bank-top sediment and rapid progradation of the slope during the Holocene along the leeward slope of the western Great Bahama Bank (Wilber et al., 1990). Variations in the relative position in sea level during the last 18 k.y. (isotope Stages 1 and 2) played an important role in the development of the aforementioned slope profiles (Grammer and Ginsburg, 1992).
Purdy (1963) and Enos (1974) have provided a detailed description of the sediment facies that are found on the present-day Great Bahama Bank. Neumann and Land (1975) made some gross calculations on the production potential of the shallow-water realm and found that large quantities of fine-grained carbonate mud are exported from the platform into the basin. Studies by Boardman and Neumann (1984, 1986), Boardman et al. (1986), Milliman et al. (1993), and Robbins et al. (1997), among others, have demonstrated the production of the fine-grained sediments on the platform top and the subsequent export to the basinal realm. Sediment-charged hyperpycnal (high density) waters are able to transport the entrapped sediment over large distances (Wilson and Roberts, 1995); therefore, this process, called "density cascading," might enhance the export of fine-grained, shallow-water material from the platform top. Hyperpycnal waters are generated in shoal waters through thermohyaline processes. Climatic changes steer the possibility that rapid water-mass modifications occur, and thus create variations in the number of density cascading events that might well evolve through time (Wilson and Roberts, 1995).
The mineralogy of the sediments on the platform top is mainly dominated by aragonite (e.g., Pilkey and Rucker, 1966; Milliman, 1974; Droxler, 1984). The variations in sediment export toward the basin will, therefore, display themselves as variations in the input of aragonite. Studies by Droxler et al. (1983), Boardman and Neumann (1984), Droxler et al. (1988), and Reijmer et al. (1988) clearly demonstrated this principle in cores taken in the Tongue of the Ocean and Exuma Sound. In addition, these periplatform sediments show a good agreement between the oxygen isotopes and the carbonate mineralogy (e.g., Droxler et al., 1983). This link might then provide us with a correlation to the observed climatic changes for these surroundings as described by Kroon et al. (Chap. 2, this volume).
The succession researched in this paper is deposited in a time slice in which the Great Bahama Bank shows an overall progradation pattern clearly visible in the regional seismic line known as "the Western Line" (Eberli and Ginsburg, 1987, 1989). Shallowing-upward trends found in the sediments recovered in the boreholes CLINO and UNDA (Eberli, Swart, McNeill, et al., 1997) confirm this general trend. It is, therefore, plausible that Sites 1003 and 1006 described in this paper were influenced more and more by the advancing platform. In connection with increased production on the platform top, this would lead to increased off-bank transport and deposition, which would invariably play an important role in the sediment development of the slope.
This paper will concentrate on variations in the mineralogy and grain-size distribution at two sites on the leeward side of the Great Bahama Bank. Site 1003, situated proximally to the platform, is positioned above thick lower slope sections and is, therefore, an ideal place to evaluate lowstand vs. highstand input signals. The distal site, Hole 1006A, then provides a more pelagic reference signal for this lowstand vs. highstand variations. As already demonstrated by Westphal (1997) and Westphal et al. (1999), the grain size in combination with the mineralogy plays an important role in the diagenesis pattern that develops within the sediments. The initial permeability seems to influence the nature of the diagenesis (Westphal, 1997). Slope sediments from the Great Bahama Bank (the CLINO core; see Eberli, Swart, McNeill, et al., 1997) showed that coarse-grained deposits with their initial high permeabilities were subjected to intense diagenetic alteration. Unlike the coarse-grained lowstand sediments, the fine-grained highstand deposits were protected against large initial fluid-flow alterations because of their low permeability (Westphal, 1997). The way in which the mineralogy and grain-size distribution is developed through time and space along the platform slope will be of great interest for later diagenetic studies.
As will be shown, both mineralogy and grain-size distributions are good proxies to determine environmental variations through time. The analysis of the different sedimentation patterns developed during glacial and interglacial periods (interglacials) might help us to understand in detail the response of the shallow-water production area of the Great Bahama Bank to changes in climate.
As mentioned above, variations in the mineralogy and the grain size will modify the diagenetic potential of the sediments. It is important to understand what types of patterns develop on a vertical (time) and a lateral (spacial) scale. Understanding these patterns in the present-day environment might then help us when analyzing ancient platform-to-basin transects.